1. Field of the Invention
This invention relates to the display of information of a cathode ray tube (CRT) monitor display. In particular the invention pertains to apparatus which permits this display controller which generates the dot pattern comprising the characters of information to be displayed on the CRT screen to be remotely located from the display monitor containing the monitor electronics and CRT.
2. Description of the Prior Art
Information is normally displayed on the cathode ray tube of a display monitor by selectively energizing an electron beam as it scans the sensitized screen of the CRT. The electron beam normally scans the screen from left to right in a succession of horizontal scan paths which begin at the top of the screen and end at the bottom of the screen. The beam is subsequently returned to the top of the screen for the next successive raster scan of the entire screen. This is accomplished by monitor electronics, or beam drive circuitry, associated with the cathode ray tube which magnetically deflects the beam in both the horizontal and vertical directions and selectively energizes the beam as it scans the screen of the CRT. The horizontal retrace of the beam is initiated by a horizontal synchronization (sync) signal, the vertical return of the beam to the top of the screen in initiated by a vertical sync signal and the beam is selectively energized in response to a video signal. These signals, the horizontal sync, vertical sync and video, are generated by the display controller and transferred to the monitor electronics which in turn uses them to generate the signals which drive the CRT electron beam gun and beam deflection magnets. Because of the high voltage of the beam signals involved, it is current practice to package both the monitor electronics and the cathode ray tube in close proximity and to refer to the combination of monitor electronics and CRT as the display monitor.
The display controller generates the horizontal sync, vertical sync and video signals by scanning a refresh memory in the display controller that contains the information which is to be displayed on the CRT screen. The video signal is generated by the display controller scanning the refresh memory a character at a time as each line of information is displayed on the CRT screen. The information within the display controller refresh memory may have originated from a keyboard attached to the display terminal, from a computer attached to the display controller, or remotely over a communication line attached to the display controller.
In many present applications of CRT monitor displays, it is desirable to be able to remotely locate the CRT monitor display from the display controller. Examples in which the CRT's monitor displays are remotely located include flight information at airports in which the CRT may be located several thousand feet away from the display controller to examples of computer consoles in which the CRT display may be located 3 or 4 feet from the display controller. One current method of remotely locating CRT monitor displays is to transmit a composite signal over a coaxial cable from the display controller to the monitor electronics. This is accomplished by having a modulating, or mixing, circuit at the display controller modulate the horizontal sync, vertical sync, and video signals to be different DC voltage levels on the coaxial cable. At the monitor electronics end, a demodulating, or stripping, circuit demodulates the composite signal and separates it into the horizontal sync, vertical sync and video signals which are in turn used as inputs to the monitor electronics. Although this system works well and can be economically employed for even those cases in which the monitor is located within 3 to 4 feet of the display controller, which is beyond the 1 to 2 foot range in which TTL level signals can be driven without encountering problems, the modulation and demodulation circuits required for the composite signal become more complex when used for other than single intensity monochrome monitor displays.
In single intensity monochrome monitor displays, the horizontal sync, vertical sync and video signals can be mutually exclusive and the modulating circuit need only integrate 3 separate voltage levels on the coaxial cable and the demodulating circuits need only detect 3 voltage levels in the composite signal to strip out the horizontal sync, vertical sync and video signals. These signals are mutually exclusive if: the horizontal retrace is initiated by the horizontal sync signals transitioning from the low to high state; the vertical retrace from the bottom of the screen to the top of the screen is initiated by the vertical sync signals transitioning from the low to high state; and the electron beam is energized when the video signal is in the high state; and only one of these signals is in the high state at any one time. That is, that during the horizontal retrace there is no video signal in the high state, nor is there a vertical retrace initiated, during the vertical retrace there is no horizontal retrace initiated nor video signal in the high state, and during the scan of a line in which the video signal alternates between high and low, depending upon whether the electron beam is to be energized or not, there is no horizontal retrace nor vertical retrace initiated. Therefore it can be appreciated that only three distinct DC voltage levels need be modulated onto the coaxial cable to compose the composite signal and at the other end the three distinct voltage levels may be simply demodulated to strip out each of the three signals.
Unfortunately, the modulating and demodulating circuits used with the composite signal become more complex when multiple intensity or multichrome CRT monitor displays are employed. For example, if the CRT monitor display is used to display information in both a high and low intensity, an intensity signal must be added to the composite signal. The intensity signal is in the high state to indicate that the character of information should be displayed in the brighter dots on the screen and the intensity signal is in the low state to indicate that the character should be displayed in the lower intensity (brightness) dots on the CRT screen. In this case, the signals which compose the composite signal are no longer mutually exclusive in that the intensity signal will be in the high state whenever the video signal is in the high state to generate a dot of a character which is to be displayed at the high intensity. This non-exclusitivity between the video and intensity signals requires that the signal modulating and demodulating circuits become more complex in order that the synchronization between the intensity and video signals is maintained within acceptable limits so that the level of the intensity signal is established in synchronization with the beginning and end of video signals for each character to be displayed on the CRT screen in high intensity.
The instant invention is directed to achieving an improved apparatus which will permit the display monitor to be remotely located from the display controller and will satisfy all electrical and synchronization requirements of the application and which will result in substantial reduction in manufacturing cost.